Heavy or settlement-sensitive facilities that are located in areas containing soft or weak soils are often supported on deep foundations. Such deep foundations are typically made from driven pilings or concrete piers installed after drilling. The deep foundations are designed to transfer structural loads through the soft soils to a more competent soil strata.
In recent years, aggregate piers have been used increasingly to support structures located in areas containing layers of soft soils. The piers are designed to reinforce and strengthen the soft layers and minimize resulting settlements. Such piers are constructed using a variety of methods including drilling and tamping methods such as described in U.S. Pat. Nos. 5,249,892 and 6,354,766 (“Short Aggregate Piers”), driven mandrel methods such as described in U.S. Pat. No. 6,425,713 (“Lateral Displacement Pier”), and tamping head driven mandrel methods such as developed by Nathanial S. Fox and known as the “Impact Pier” and described in U.S. Pat. No. 7,226,246.
The “Short Aggregate Pier” technique referenced above, which includes drilling or excavating a cavity, is an effective foundation solution, especially when installed in cohesive soils where the sidewall stability of the hole is easily maintained.
The “Lateral Displacement Pier” and “Impact Pier” methods were developed for aggregate pier installations in granular soils where the sidewall stability of the cavity is not easily maintained. The “Lateral Displacement Pier” is built by driving a pipe into the ground, drilling out the soil inside the pipe, filling the pipe with aggregate, and using the pipe to compact the aggregate “in thin lifts.” A beveled edge is typically used at the bottom of the pipe for compaction.
The “Impact Pier” is an extension of the “Lateral Displacement Pier.” In this case, a smaller diameter (8 to 16 inches) tamper head is driven into the ground. The tamper head is attached to a pipe, which is filled with crushed stone once the tamper head is driven to the design depth. The tamper head is then lifted, thereby allowing stone to remain in the cavity, and then the tamper head is driven back down in order to densify each lift of aggregate. An advantage of the Impact Pier, over the Lateral Displacement Pier, is the speed of construction.
The invention is an improvement on such prior art techniques, and in particular, the Lateral Displacement Pier, Impact Pier and their methods. A more efficient mechanism is provided for compacting aggregate by restricting upward movement of the aggregate through the mandrel during driving of the mandrel.
Generally, the invention employs a steel mandrel made up of an upper pipe as a primary portion used for the delivery of aggregate to a lower pipe portion or tamper head. During extraction of the mandrel, upward movement of aggregate is minimized. However, during compaction there is a possibility that materials may be pushed up into the mandrel as the mandrel is forced down. In accordance with the invention, the possibility of materials moving up into the mandrel is eliminated or substantially reduced.